The present invention relates to systems for electro-optic sampling of electrical signals being analyzed and to improved construction of Pockels cells included therein.
An electro-optic sampler is especially suitable for measurement of picosecond transient signals such as those produced by photodiodes, integrated circuits and other fast devices which either have an electrical stimulus and electrical output or which have an optical stimulus and an electrical output. Where the charactertics of the system are known the electrical characteristics of a device-under-test can be measured.
Electro-optic sampling systems of the type contemplated within the text of the present invention utilize Pockels cells. A Pockels cell comprises what is referred to in the present specification as an electro-optic crystal which has the property of variable birefringence as a function of electrical field applied thereto. This effect was described in F. Pockels, Lehrbuch der Kristalloptic, (Teubner, Leipzig, 1906). The electro-optic crystal is an expensive element of an electro-optic sampler. In accordance with the present invention, usability and reusability of electro-optic crystals is faciliated so that availability of use of electro-optic sampler can be expanded.
The electro-optic crystal is utilized in the following context. An optical pulse train is provided from a source and split into two different paths, a sampling beam and stimulus beam. One such source is a visible wavelength picosecond laser. Optical pulses in the first path trigger generation of the electrical signal to be measured. This electrical signal is coupled to be accessible to the electro-optic crystal, through which optional sampling pulses of the second path are propagated. The crystal is in an optical path between first and second crossed polarizers. The field-induced birefringence varies the polarization of the sampling beam. The sampling beam intensity after polarization analysis is measured by a detector, e.g. a slow photodiode, one which does not have to resolve individual pulses.
The detector output is provided to utilization means. Electrical output from the detector as well an electrical output indicative of modulation of pulses in the stimulus beam are first coupled to a lock-in amplifier which yields a DC output proportional to the amplitude of the sampled electrical signal in phase with the modulation of the stimulus beam. A display can be generated by plotting the output of the lock-in amplifier during successive pulse periods against the output of a variable delay line synchronized with the display device. The basic theory of electro-optic sampling is explained in J. A Valdmanis and G. Mourou, "Electro-Optic Sampling: Testing Picosecond Electronics", Laser Focus/Electro-Optics, February, 1986, Page 84 and J. A. Valdmanis, G. A. Mourou, and C. W. Gabel, IEEE Journal of Quantum Electronics, Volume QE-19, 4, Apr. 1983, p. 664. An effective electro-optic sampler for measuring signals having temporal components on the order of picoseconds is disclosed in U.S. Pat. No. 4,446,425 issued on May 1, 1984 to J. A. Valdmanis and G. Mourou.
In the most common implementation of electro-optic sampling, the electro-optic sampler is embodied in a test fixture composed of three parts. These are a metal or ceramic carrier, a photoconductive switch and an electro-optic crystal. The carrier provides mechanical support for active devices. The active devices include the electro-optic crystal itself, the photoconductive switch and the device-under-test. The photoconductive switch is typically a chip of GaAs a few millimeters square with a metallic wave guide of sub-millimeter dimensions deposited on surfaces thereof. The photoconductive switch is attached with adhesive to one end of the carrier. The electro-optic crystal may typically comprise lithium tantalate and has a transmission line such as a waveguide deposited on its surfaces. It is attached to the other end of the carrier with an adhesive. The device-under-test is attached to the carrier, also with adhesive, between the switch and the crystal. Electrical connections are made from the device-under-test to the wave guides on the switch and on the crystal as well as to a bias network typically with gold wire bonds. In the operational mode, the photoconductive switch has appropriate bias applied thereto. When it is stimulated with the stimulating beam described above, an electrical pulse with picosecond risetime is launched down the waveguide. This is the stimulus signal which stimulates or turns on the device-under-test. The device-under-test produces an electrical output pulse which is then launched down the waveguide on the crystal surface where its electrical field affects the birefringence of the electro-optical crystal and is sampled by the second train of optical pulses.
A problem arises when testing is complete and another device is to be tested. The device-under-test generally cannot be removed from the fixture without destroying the electro-optic crystal since the materials involved, typically lithium tantalate are fragile. Removal of the wire bonds from the waveguides on the electro-optic crystal could severely damage these waveguides. Adhesion to crystal surfaces can qualitatively be described as only being fair. Bearing in mind the fragility, very small size and cost of rework on the electro-optic crystal in particular frequent replacement of electro-optic crystals is not feasible for most users.
Prior schemes which have been proposed to avoid these problems involve the principal of eliminating the permanent physical connection between the device-under-test and a separate electro-optic crystal. These methods require that the crystal be brought within a few microns of the device-under-test. If the crystal touches either the substrate or device-under-test with more than a minimal amount of force, e.g. ten grams, either would likely be destroyed. Another proposed solution is the fabrication of the device-under-test on a substrate exhibiting a Pockels effect, using the device/substrate itself as the electro-optic crystal this allows non contact probing, but without the sensitivity, calibration and position control problems of other non-contact schemes. This technique is not useful with respect to silicon devices since silicon does not exhibit the Pockels effect.
Another approach that has been considered by some is the use of connectors to permit insertion and removal of the device-under-test from the signal path. No commercial connector currently available has a bandwidth beyond 50 GHz, and it appears that extending this mode-free bandwidth almost an order of magnitude is unlikely. Even coplanar waveguides of sub-millimeter dimensions have sufficient dispersion to increase the risetime of a one picosecond signal to about ten picoseconds after less than five millimeters of propagation distance. Waveshape distortion accompanies this effect. Another variation of a non-contact concept utilizes a thin electro-optic crystal with a dielectric, high reflection coating on its bottom surface placed near or in contact with the top of the coplanar output waveguide. Electric field propagating down the waveguide penetrates the crystal, and sampling pulses reflected from the bottom surface are used to make the measurement. This technique avoids many of the problems mentioned above but continues to suffer the disadvantages of requiring sophisticated positioning apparatus. This also requires use of the reflection mode. As further described below, it is desirable to be able to use either a reflection mode or transmission mode. In accordance with the present invention, it is desirable to provide for repetitive measurements of devices-under-test with the use of a single electro-optic crystal which may be dedicated to a particular test system.